Not correct about set the direction and forget it LOL
because a spacecraft can not operate on just 1 thruster.
That would make impossible to change direction. So there are more than 1 thruster. Observe drones. they usually have 4 fans for lift and direction control. Helicopters have 2 fans.
The thrust is never exactly constant because fuel or any other propulsion system never burns at extreme steady state. That is why there is gyroscopic for directional control.
As I explained before, the target 4+ light years away is smaller than a needle point when the spacecraft starts out. No way to set it and forget it. It will need constant direction control for years to come. Even flying to the moon just a few thousand miles away it is not set it and forget it.
As for flying to the moon, there's gravity from two bodies as well as acceleration, deceleration, etc.
Bridging the gap from one solar system to another has no forces other than momentum - no acceleration, no deceleration, and only the gravity of the destination solar system. We can calculate the consistent orbits of 100-year comets and they're exposed to gravity from multiple celestial bodies. Plotting and maintaining a course between two solar systems would be less difficult.
There's also stored power, and the potential to have an array that can harness power from cosmic rays.